Since the continuous miniaturization of the feature size in integrated circuit (IC) technology and the scaling down of the size of the devices, as well as the increasing amount of transistors, allows IC to be operated at high speeds with low power consumption and the significance of the electromagnetic compatibility (EMC) of IC is increased. The high performances desired of the IC not only produce noise, but also make the IC itself sensitive to interference. This situation leads the need for characterizing their behaviors of emission and immunity. To investigate these problems, several measurement methods have been developed as the standards.
According to the transfer types of electromagnetic waves, the test methods can be further classified into radiated or conducted methods. A concise method to characterize the conducted emission of IC is known as the 1Ω/150Ω direct coupling method. The direct coupling method guarantees an IC electromagnetic emission (EME) measurement with high repeatability and correlation. Since EME is caused by fast changes of currents/voltages inside the IC, the resulted radio frequency (RF) currents/voltages distribute and form emitting loop antennas via on-chip passive distribution networks (PDN). In order to analyze the RF currents/voltages, two acquisitions named 1Ω current measurement and 150Ω voltage measurement are specified herein. Probes composed of few lump components are used to make the observation on EME behavior of a certain IC pin easier.
All RF currents form at least one loop which flows out and back to the IC. The return paths are mostly via the ground or the power plane. Therefore, the ground pin of IC is a great position for measuring the RF return current. FIG. 1 is a circuit view illustrating a conventional test system. As is shown, a 1Ω probe 102 is inserted between the ground pin 106 of the IC 104 and the ground 108 to measure the RF voltage across the 1Ω resistor of the probe. The RF voltage from all of the RF currents returning to the 1Ω is measured by a test receiver 110. As shown in FIG. 2, the 1Ω probe 20 is composed of a 1Ω resistor 202 and a 49Ω resistor 204. One end of the 1Ω resistor 202 is linked to the ground pin of the IC and the other end is connected to ground. The 49Ω resistor 204 is placed between the ground pin of the IC and the test receiver with 50Ω input impedance. As a result, this configuration achieves 50Ω (49Ω plus about 1Ω) impedance matching, which satisfies maximum power transmission when viewed from test receiver side. From the ground pin side, the 1Ω probe provides a low impedance current path for IC operation.
Due to the miniaturization trend in electronic devices, surface mounted devices (SMDs) have become the best candidate to realize a printed circuit board (PCB) level design. The leadless property reduces unwanted parasitic effect more than an axial leaded device for high frequency or high speed applications, but the parasitic effect never disappears. A typical high frequency model of an SMD resistor is shown in FIG. 3. The resistor 302 represents the intrinsic resistor and the inductance 304 forms by the finite length of the resistor and contacting pads, while the capacitance is the coupling of the pads. These parasitic values could be measured or provided by the manufacturers. Unfortunately, unlike the inductors and capacitors which are used frequently in RF and microwave applications, most resistor data sheets do not provide detailed models or frequency responses of these parasitic components. Sometimes only limited information, such as low band results, can be obtained, or some other insufficient results were released for estimation. Especially for a low value resistor, the inductance dramatically dominates the impedance out of the low frequency band. Therefore, the estimation of choosing components relies on basic network analysis. By measuring the S-parameters of the resistor, the real part and imaginary part can be distinguished. Then user can choose the appropriate component from different vendors to implement into the design.
Accordingly, a need has arisen to design the circuit of the 1Ω probe, which can eliminate the effect of the parasitic inductor of the equivalent circuit without relying on the resistor information provided by the suppliers.